Activation of monocyte-derived cells occurs at most sites of inflammation and serves as a first line of defense in the immune system by removing foreign objects and by signaling to other immune cells. Activated macrophages are now known to be heterogeneous populations of cells that can mediate both pro-inflammatory (classically-activated macrophage, or M1 cells) or pro-healing (alternatively-activated macrophage, or M2 cells) behavior. It has been shown that cytokines expressed by M2 cells help to regulate M1 behavior by moderating the inflammatory response. Thus, the balance between M1 and M2 cells is hypothesized to be important in the normal healing response. Recently, heightened activity from M1 or M2 cells has been associated with several disease states, including fibrosis, atherosclerosis, and tumor progression. Interestingly, recent studies indicate plasticity even in differentiated macrophage cells, including evidence of repolarization of activated macrophage. Bioengineering technologies that identify harness and direct specific subpopulations of monocyte-derived cells are therefore critical tools in the development of cell-based therapy approaches addressing the aforementioned diseases. The overall goal of this proposal is to develop M1- and M2-specific peptides that can be used as cell markers or as targeting ligands to deliver modulating drugs to sub-populations of activated macrophages. This goal will be achieved by first identifying M1- and M2-binding peptides from combinatorial libraries by sequential negative and positive biopanning. Potential M1- and M2-peptides will be synthesized and tested by flow cytometry and confocal microscopy. Binding peptides will then be conjugated to quantum dots to serve as cell specific markers and assessed in mixed cell populations and in tissue sections. In addition, peptides will be conjugated to nanoparticles and evaluated for targeted delivery to sub-populations of activated macrophages. Finally, the cross-species recognition of M1 and M2 cells by these peptides will be assessed in non-human primate and human tissue sections. Public Health Relevance: An imbalance in subpopulations (M1 and M2) of activated macrophage cells resulting in heightened or prolonged activity from either M1 or M2 cells has been hypothesized to contribute toward diseases such as fibrosis and atherosclerosis. The goal of this work is to develop M1- and M2-specific peptides that can be used to identify, purify, or target these macrophage subpopulations. Bioengineering technologies that identify, harness and direct specific subpopulations of monocyte-derived cells are critical tools in the development of cell-based therapy approaches addressing the aforementioned diseases.